Electrooptical transmission system



Oct. 12, 1937.

P. M ERTZ ELECTROQPTICAL TRANSMI SS ION SYSTEM /A/VEA/TOR P. ME?? TZ 5r Hx A u we. interi/relu',

Patented Oct. l2, 1937 UNE-'EEE STATES Search PATENT ELECTROOPTICAL TRANSNEISSION SYSTEM Application June 2, 1934, Serial No. 723,735

1 Claim.

This invention relates to electro-optical transmission and more particularly to a method and means for reducing mutual interference between television signals transmitted respectively over different adjacent or nearby circuits. This nterference is ordinarily referred to as cross-talk, a term borrowed from the telephone art.

An object of this invention is to reduce the mutual interference or cross-talk between two or more electro-optical systems operating either in the same or opposite direction.

When an object or eld of view is periodically scanned in a series of adjacent parallel lines and the light tone values of elemental areas are translated into electric current variations the energy is largely concentrated in a number of distinct bands of frequencies between which there very little useful energy, and theposition of the bands in the frequency spectrum is dependent upon the eld scanning frequency and upon the line scanning frequency. I'he energy concentrations resulting from scanning ordinary fields occur in the regions of the field scanning frequency and some of the lower harmonics thereof and in the regions of the line scanning frequency and the lower harmonics thereof. These latter bands are made up of a plurality of frequencies having a difference of approximately that of the picture scanning frequency and ordinarily may have, for example, a band width of approximately 20 per cent of the line scanning frequency in which case the low energy intermediate gapsor valleys have a frequency band width of approximately per cent of the line scanning frequency. In other cases, the band width of the energy concentrations may be much wider, particularly in telephotography. The width of these bands are governed more or less by the filtering action of the scanning aperture and by the characteristics of the object or eld being scanned.

This invention provides means for utilizing the above mentioned low energy gaps or valleys for reducing mutual cross-talk between two or more systems simultaneously operating by introducing a relative shift in the frequencies of the several signals so that the energy peaks of each system come in the low energy regions of each of the other systems and vice versa.

In accordance with the form of this invention herein shown as illustrative of the invention, means are provided for simultaneously operating with the saine resolution two electro-optical transmission channels either in the same or in opposite directions. Two separate parallel television systems are shown Calin CmplOyng carrier our rents for the transmission of the electro-optical signal. 'Ihe carrier currents dier in frequency by one-half the line scanning frequency oran odd multiple thereof, so that the signals sent out have a relative shift in frequency by this amount. More than two signals may be simultaneously transmitted in which case the carrier currents are such as to produce relative frequency shifts in the signals to cause their energy concentration to occupy mutually exclusive frequency positions.

By this arrangement, the far end cross-talk requirement between several systems operating over circuits of a pole line or cable may be made much more lenient than in the ordinary arrangement where lthe carrier currents have the sam frequency. v l n This intention is applicable to tele-photography as well as to television but in the case of the former the practical details of the application are more diuicult because of the very close spacing of the practically idle and thel utilized frequency bands.

In this arrangement the over-all range of the frequency band width of the several signals con' currently transmitted in each case is substantially the same as if only one signal were transmitted. The relative shifting of the energyconcentrations of 4the signals in accordance with this invention is quite different from that of utilizing the low energy gaps of a given signal for the transmission of additional signals over the same circuit for the purpose of making greater use of the availfollows and is illustrated in the accompanying.A

drawing: v

Figure l is a typical current-frequency diagram of the energy concentrations of two photoelectric signal currents generated from scanning two ordinary objects in a sericsof parallel lines, each at a line scanning rate .of approximately C cycles per second and combining one or both of the photoclectric currents so generated with a iiocm carrier current o: currents, respectively, of con- 5ta/.llt frequency to produce a combinatlcnfrequenoy or combination frequencies, respectively, which are shifted so that the energy concentra.- tions of the two photoelectric signal currents are relatively shifted by one-half the line scanning frequency;

Fig. 2 is a general schematic representation of two separate electro-optical systems transmitting in the same direction and so arranged that the energy peaks in one system come in the regions of the energy valleys of the other system and vice versa;

Fig. 3 is a general schematic representatie similar to that shown in Fig. 2 but arranged for transmitting in opposite directions; and

Fig. 4 diagrammatically shows the essential apparatus elements and their interconnections for a single circuit one-way transmission television system adaptable to this system employing a plurality of circuits.

The typical current-frequency diagrams A and B shown in Fig. 1 show the signalenergy concentrations for two photoelectric currents relatively displaced by one-half the line scanning frequency. This invention is concerned primarily with the relative displacement of the energy concentrations and one of the signals need not have its energy concentrations shifted. However, it may be desirable to displace the energy concentrations of all signals somewhat to better adapt the frequency bands of the signals to the transmission characteristics of the available circuits. Certain circuits are not adapted to transmit direct current or the lower frequency components and therefore a frequency displacement of all signals upwardly so that the lower frequencies thereof are high enough to avoid transmission difficulties may be desirable thus placing all of the signals in substantially equally favorable positions with respect to the transmission characteristics of the several circuits. Considering the signals as having a line scanning frequency of 1000 cycles per second and a total band frequency width of 20,000 cycles, two signals may be displaced relatively to each other 500 cycles, namely, one-half the line scanning frequency by combining one of the signals with a carrier current of 500 cycles per second and using the upper sideband thereof or, preferably, by combining with a carrier current of 20,500 cycles and using the lower sideband thereof. This would place the displaced signal energy concentrations of one of the signals of the signal frequency spectrum in the regions of the odd multiples of 500 cycles', namely, at 500, 1500, 2500, etc. to 20,500, while the energy concentrations of the other signal would remain inl the region of the even multiples of 500, namely, zero, 1000, 2000, etc. to 20,000. If it is desirable to effect frequency displacements of respectively :I: and :c plus one-half the line scanning frequency, as indicated in Fig. l, in the signals, respectively, to better adapt them for transmission over the available circuits, the other signal would ne similarly combined with a carrier current of 1000 or of 21,000 cycles per second and either the upper or the lower sideband used for transmission. The displaced energy concentrations of both signals would still occur around the position of the odd and the even multiples of 500, respectively, but would commence in the region of 500 and 1000 cycles per second, respectively. Where three signals are employed, the energy concentration of one signal would be displaced one-third the line scanning frequency from the corresponding concentrations of the other signals.

The frequency band positions of the several signals with reference to the useful frequency band of the transmission media may be such as to clear any desired portion of the latter for the transmission of other signals. For example, the television signals might all have an upward displacement :c of at least 3,000 cycles and all be transmitted above 3,000 cycles, thus permitting this cleared low frequency range to be used for sound transmission, synchronizing, monitoring,

etc.

The above .ldescribed plan of symmetrically positioning the` energy concentration of the different signal currents in more or less mutually exclusive frequency positions is limited to a comparatively small number of relative displacements owing to the fact that the energy concentrations have a substantial frequency band width and if this plan were carried on to the point where appreciable overlapping occurred-its effectiveness would be decidedly decreased. However, even for the simultaneous transmission of a larger number of signals a real advantage in cutting down cross-talk may be obtained by relatively displacing the energy concentrations as between different groups of circuits. For example, considering twelvevoircuits between which cross-talk may occur, six of them may transmit a signal without relative frequency displacement of the energy concentrations of the signals with respect to each other, and a second group of six.

circuits may be so operated but with a frequency displacement as between the two groups, the displacement of the energy concentrations of one group with respect to those of the other group being one-half the line scanning frequency. This group arrangement might be carried scmewhat further and thc twelve. circuits placed in different groups of four circuits each with the relative frequency displacement as between the several groups equal to one-third the line scanning frequency. When two circuits or two groups of circuits have relative frequency displacement of their signal energy concentrations,

'the arrangement is somewhat more effective in the case when half of the circuits are transmitting in one direction and half in the opposite direction in which case near end cross-talk. occurs as between the groups and far end crosstalk occurs within each group. `Where all are transmitting in one direction, the cross-talk obviously occurs at the far end. When the transmission is in opposite directions, cross-talk is more acute owing to ,the fact that the energy level of the signals being transmitted is higher than that of the signals being received.

Fig. 2 schematically shows two television transmitting systems A and B, respectively, arranged for the energy concentrations of the signal current of one system to occupy the regions of the energy valleys of the signal current of the other system. The transmitters IDO--A and IDG-B generate the respective signal currents which, by means of modulators IIC-A and liti-B are combined with suitable carrier currents generated by oscillators |20-A and H20-B. respectively. The respective modulated carrier current signals are transmitted over suitable individual transmission circuits and demodulated by demodulators ZID-A and ZlU-B at the .respective receiving stations. Television receivers 20D-A and 20G-B operating in Asynchronism with transmitters |0-A and H10- 5, respectively, reproduce the images in the usual manner. The carrier current frequency generated by the we. interim?,

Search Root oscillators I20-A and IZB-B, respectively, differ in frequency by one-half the line scanning frequency, or perhaps by a different fraction thereof if more than two circuits are considered.

While Fig. 2 shows an arrangement for both circuit -with the cathodes of the vacuum tubes and the other at the midpoint between the two inductances. Transformers may be used instead of inductances. In either case, the two output coupling units should have separate cores, while the input coupling units may have a common core or separate cores. If double-balanced demodulators are used, they would comprise two units slmilarto those shown forming the simple over the transmission paths of one group in one direction and the other group in the opposite direction.

PIERRE MERTZ.

circuits to transmit in the same direction, the demodulator 2li) connected in parallel. An ad- 5 transmission may be concurrently in opposite divantage of the double-balanced demodulator is rections as schematically shown in Fig. 3. The that it eliminates more of the extraneous fredescription of Fig. 2 is applicable to Fig. 3 as quencies than the simple demodulator and it is the apparatus elements are similar and the cortherefore preferable in circuits demodulating responding reference characters are identical. television currents. The television receiver 200, Fig. 4 diagrammatically shows the essential including the usual current amplifying apparatus,

apparatus elements and their interconnection for may be of the usual type operated in synchronism a television system transmitting over one transand in phase with the television transmitter 00. mission circuit in one direction adaptableto this The apparatus and its interconnections for a system employing a plurality of circuits. The plurality of television transmission circuits artelevision transmitter [00 including the usual ranged for operation in accordance with this incurrent amplifying apparatus generates the Vention is similar for each circuit to that shown photoelectric signal current by scanning the eld in Fig. 4. However, in the operation of different of View in successive line series of elemental circuits such carrier frequencies are chosen as 2o areas. The amplified signal is impressed upon will produce the desired relative displacement of the circuits of the modulator H0 lthrough any the energy concentration in the several signals or suitable coupling such as transformers H3 and groups thereof. The diierent frequeney Stated H4. The modulator may be a simple modulator in describing this system. for simultaneous transof the push-pull type as here shown or a doublemission of a plurality of television signals are balanced modulator comprising two units similar typical and are mentioned primarily to facilitate to those of the simple modulator suitably interthe description. Obviously, different Scanning connected. The simple modulator HD comprises l rates may be selected Which Would Cause energy vacuum tube elements IH and H2 connected in Concentrations to OCCur at different freCluenCy push-pull arrangement. The incoming photopositions in the television signal current specelectric signal current is impressed upon the grids trum. The energy Conoentretions of the signal of these tubes by means of transformers H3 and current as transmitted over the diierent respec- H4 while the carrier current generated by the tive Circuits may not only be staggered or disoscillator |20 is supplied through the transformer placed but may be positioned Within any desired y I 2l. The modulated outgoing signal current is over-all frequency bend so as to permit the use 30 transmitted through the transformers H5 and 0f Certain frequeney bends for' the transmission HE to the transmission circuit. output of other si'nf/ils 'Which it m13' hn t transformers should have separate cores while have aooompeny the television transmission. the input transformers may have either separate Preferably only one side band 0f the Carrier Sgor combined Cores, If a, double-balanced modunal currents would be transmitted in order that 40 lator which introduces further renement is dethe tot-a1 frequency Width of the signals transsired, it may be formed, as stated above, by using mitted muy be es nliOW es pOSSiblG. The Certwo units similar to those shown for modulator Tier Current may be transmitted as is obvious H6 connected in parallel. The double-balanced from the arrangement Shown in Fig. 4 o1' it may modulator has the advantage over the simple be suppressed and supplied at the receiving sta- 45 modulator of cutting out more of the extraneous tion from a local source. All of these variations frequencies produced in modulation. It may be are. in general, Well known to the art and this desirable to insert a low pass nlter between the invention relates primarily to the operation of a output transformers of I the modulator and the number of television Systems in a manner to retransmission line to eliminate the undesirable duce cross-talk between their respective circuits high frequency components passed by the moduto a minimum. lator but usually this will be unnecessary since What is claimed is: the output side of each modulator is connected Means for image Current transmission comto an individual transmission line which will Dlising a plurality of Systems having indiVidual largely prevent undesirable high frequency com- -physieal transmission paths, said paths being so 55 ponents reaching the receiving station. At the Closely oSSOCiated that energy transmission suffireceiving end, the incominor signal including its cient to produce cross-talk may take place, means carrier is impressed upon the demodulator '2|0. including scanning devices for generating image Any suitable amplifying apparatus, though not currents in a group of such paths so that correshown in the drawing, may be employed to amsponding energy concentrations have similar poplify the incoming signal. The demodulator may sitions-in the frequenoy SDeCtrum in each image be either a simple demodulator as here shown or Current Within the group, means including scana double-balanced demodulator. The simple dening devices for generating image currents in a modulator 2li! comprises vacuum tube elements second group of Seid DthS S0 that Correspond- 2H and 2l2 connected in push-pull airangeing energy concentrations have similar positions 65 ment. The input circuit is connected with the inithe frequenCy speetriun in the image currents incoming transmission line through transformers transmitted over the second group 0f paths but 2l3 and 2M and the output circuit of kthe dedifferent positions as between the rst and second modulator is obtained by using inductances 2|5 groups of image currents, and means for simuland 2|$ and connecting one side of the output taneously transmitting the generated currents 70 

